/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function(digits) {
  const arr = ['', '', 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz'];
  function backTrack(dight, ) {

  }
  for(let i = 0; i < digits.length; i++) {

  }
};
let digits = "23";
console.log(letterCombinations(digits));
digits = "2";
console.log(letterCombinations(digits));
